The DGS-method (DGS=Distance, Gain, Size) has been known for a long time from the prior art. The DGS-method has been originally developed for plane circular straight beam probes generating a rotational symmetric sound field in the test material. It is based on the comparison of the echo amplitude of a real reflector in a workpiece (for example, of an inclusion, a cavity, a crack, etc.) with echo signals of circular disk reflectors. With the aid of a so-called DGS-diagram, an inspector can determine a equivalent reflector size (ERS) characterizing the real reflector through the comparison of the echo amplitude of a real reflector with an array of curves of circular disk reflectors recorded in the DGS-diagram. For this he selects the device gain, so that a reference echo (as a rule a back wall echo) attains a pre-set screen height. Also, the reflector echo is adjusted by means of the gain adjustment to the same screen height. The gain difference between reference and reflector echo, as well as the sound paths go into the DGS-evaluation. Often the DGS-method is used for sizing reflectors in the workpiece, which, for example, exceed recording limits pre-set by technical standards (see for example EN 583-2). Further details regarding the non-destructive testing of a workpiece by means of ultrasonic sound, in particular by means of the DGS-method, can be learned from the book J. Krautkrämer, H. Krautkrämer, Materials Testing with Ultrasonic Sound, 1986 (5th edition), released in the Springer Publishing House (ISBN 3-540-15754-9), here particularly Chapter 19.2.
However, in the case of recent experimental studies by means of the DGS-method, it was found that with measurements in angular intromission of sound or of an intromission of sound in the case of curved workpiece surface areas, unexpectedly large deviations resulted from the values, which had been expected for the circular disk reflectors examined. The size of the deviation depended thereby on the type of test probe utilized.